The reaction between carbon and fluorine makes it possible to obtain, in particular, solid compounds called carbon polyfluorides of general formula (CF.sub.x).sub.n in which x is a number whose average value is between 0 and 1.25 and n is an integer greater than 1.
In the present description "carbon polymonofluoride" means a carbon polyfluoride of general formula (CF.sub.x).sub.n in which x is a number whose average value is between 0.8 and 1.25.
Although the original work on carbon fluorination goes back to H. Moissan, the preparation of a product of defined composition is the work of O. Ruff, O. Bretschneider and F. Ebert (Z. anorg. allgem, Chem. 217, 1, 1934). The conditions of preparation, the structure, the properties and the conditions of utilization of graphite carbon polyfluoride are described by N. Watanabe, and T. Nakajima, in Preparation and Properties of Organofluorine Compounds, R. E. Banks, page 297, chapter 9--John Wiley--1982.
The following chemical equations express the main reactions which can take place between carbon and fluorine:
(1) total fluorination of carbon: EQU C+2F.sub.2 .fwdarw.CF.sub.4
(2) controlled fluorination of carbon: EQU n(2C+F.sub.2).fwdarw.2(CF).sub.n EQU n(4C+F.sub.2).fwdarw.2(C.sub.2 F).sub.n
(3) fluorination of carbon polyfluoride: EQU 2(CF).sub.n +3nF.sub.2 .fwdarw.2nCF.sub.4
(4) thermal decomposition of carbon polyfluoride: EQU 4(CF).sub.n .fwdarw.3nC+nCF.sub.4
Taken as a group, these reactions constitute a highly exothermic system. This is why many authors have applied themselves to a better control of the different reactions with the aim of improving the yield of formation of carbon polyfluoride.
In particular, investigations of the kinetics of carbon fluorination have been published (N. Watanbe, M. Takashima, Kogy Kagaku Zasshi, 74, 1788, 1971).
Numerous patents describe processes aimed at improving the course of the carbon-fluorine reaction by means of modifications of the operating conditions such as:
preactivation of carbon with ammonia, steam, hydrofluoric acid, fluorine when cold, sulphuric acid or by means of heat treatment,
strict control of the carbon particle size,
addition of additives to the gaseous phase (oxygen, carbon dioxide, hydrofluoric acid, carbon tetrafluoride, gaseous fluorocarbons, sulphur hexafluoride), and
addition of additives to the solid phase such as metallic fluorides like alumina fluoride and carbon polyfluoride.
Because of the difficulties encountered in controlling the reaction, non-continuous processes were essentially used until 1979. Now, any non-continuous process presents the following disadvantages:
poor removal of heat from within the carbon-containing material, whose thermal conductivity is low. (This results in poor control of the reaction temperature, which can give rise to runaways and cause uncontrolled inflammations leading to undesirable by-products.),
limitation of the reaction rate by the intergranular diffusion of fluorine, which leads to high residence times in the reaction zone and consequently to a low space time output, and
numerous and costly handling operations.
As a result, a number of authors have attempted to prepare carbon polyfluoride by means of a continuous process.
French patent application No. 80/06,889, published under No. 2,452,473 discloses a continuous process for fluorination of carbon particles in an apparatus consisting of a horizontal reactor subjected to vibrations and provided, at the two ends, of the reactor, with orifices for feeding and orifices for removing solid material and orifices for feeding and orifices for removing the reactive gas.
However, this process, as described, presents the disadvantage of difficult reaction control, owing to the inflammations of the carbon particles which generally occur. This is detrimental to the quality of the product obtained. Furthermore, the apparatus such as described, contains a high proportion of useless space. This does not permit a high space time production rate and a high fluorine yield to be obtained at the same time. Thus for a space time production rate P of the order of 10 kg.multidot.h.sup.-1 m.sup.-3, a space time production rate considered to be the best possible in the present state of the art, it is not possible to obtain a fluorine yield higher than 25%.
The space time production rate P and the fluorine yield Y of a continuous process for the preparation of carbon polymonofluoride are defined as: ##EQU1##
The process which is the subject of the invention does not present the above mentioned disadvantage. For a space time production rate P higher than 10 kg.multidot.h.sup.-1 m.sup.-3, it permits a fluorine yield Y higher than 50%, capable of exceeding 60%, to be obtained in the absence of recycling of the gases originating from the reaction zone. Considering the high fluorine concentration at all points of the gaseous phase/particles interface, it is completely surprising to obtain a fluorine yield Y as high as this. Whereas, a preferential and high formation of fluorine-rich fluorocarbon compounds such as carbon tetrafluoride could be expected.